Energy storage devices have become an integral part of our lives, but they still aren't really a part of our electric grid. There are some good reasons for that—at that scale of storage, batteries become gigantic and extremely expensive. But the lack of storage on the grid has some distinct drawbacks, putting the stability of our electric system at risk and making it harder to add in lots of renewable energy generation.

Because of that, researchers are looking for ways to get the benefits of batteries without some of the detriments. There are lots of different ways to do this, but one solution is particularly awesome to describe. Hint: It involves caves.

At any given moment, there must be almost exactly the same amount of electricity being produced as there is being consumed. If the balance tilts either way-even by a fraction of a percent-it could lead to a blackout. To simply keep the lights on, the grid has to be constantly monitored, with controllers predicting demand and making small adjustments, minute-by-minute, to supply. This happens 24 hours a day, 7 days a week.

... That's where CAES comes in. CAES systems store energy underground in the form of compressed air, but to make it work you have to start with the right kind of geology. In particular, you need a space that's airtight. This means that you can't just pump air into the sort of cave you've toured while on vacation. Instead, you have to find a hollowed-out space underground that used to hold something naturally-such as a natural gas reservoir that's had all of the gas pumped out of it.